Sexual Compulsivity (SC) is characterized by sexual fantasies and behaviors that increase in frequency and intensity so as to interfere with personal, interpersonal, or vocational pursuits. Researchers have consistently identified strong links between SC and risky sexual behavior in a variety of populations, but particularly among men who have sex with men (MSM). Sexual behavior is not the only defining feature of SC; sexually compulsive men report a series of psychological symptoms (drug and alcohol abuse, mental health and affective comorbid disorders) associated with their sexual behavior. Research suggests that these psychological factors may exacerbate HIV risk for SC MSM. However, past research on SC MSM has not distinguished between the role of behavioral versus psychological factors in HIV risk among SC MSM. Most studies compare SC MSM with other MSM without controlling for general rates of sexual behavior. Our research suggests that there are significant numbers of MSM who are behaviorally similar to SC MSM (i.e. they have equivalent numbers of sexual partners and engage in sexual behaviors with equivalent frequency), but are psychologically very different from SC MSM (i.e. they do not report distress about their sexual behavior, loss of control, that their sexual behavior interferes with their personal and professional lives, or that they engage in sexual activity under the influence of drugs and alcohol as frequently). These men - highly sexually active, but non-SC MSM - will provide a critical window into the mechanisms by which SC is associated with greater HIV risk and sex under the influence of drugs and alcohol. Identifying factors that distinguish highly sexually active SC-MSM from highly sexually active non-SC MSM will allow us to identify psychological factors that make SC men more vulnerable to high-risk sexual behavior and inform the development of HIV prevention interventions for both populations Addressing these limitations and consistent with PA-07-409 Health Research with Diverse Populations, this study will compare two samples of highly sexually active MSM, one cohort which exhibits SC symptoms and one which does not. By comparing highly sexually active SC and non-SC MSM, we can better understand both the connections between SC and HIV transmission, and the psychological, environmental, substance use, and contextual factors that make certain men vulnerable to SC and its negative consequences. We will use respondent-driven sampling to recruit 400 (n=200 SC, n=200 non-SC) highly sexually active MSM (> 9 partners, < 90 days) Participants will be followed longitudinally for 13 months and take part in a variety of assessments, capturing both antecedent conditions (neurocognitive deficits, and experienced stigma and abuse) and event-level triggers (drug and alcohol use, and affect). This study will determine the role of these factors in mediating and moderating the relationship between SC and HIV risk behavior. Our longitudinal approach will allow tracking of trajectories across groups and analysis of behaviors within individuals. Ultimately, this study will serve as the foundation for formative intervention development to reduce HIV associated risks among highly-sexually-active MSM.